The present invention generally relates to a semiconductor power converting apparatus with employment of semiconductor elements and the like. More specifically, the present invention is directed to a semiconductor power converting apparatus capable of suppressing an occurrence of an overvoltage while a switching operation is carried out.
As disclosed in IPEC2000 S-17-3 xe2x80x9cDevelopment of IGBT series and Parallel Connection Technology for High Power Convertersxe2x80x9d, each of arms of a power converter constituted by a series connection of MOS control semiconductors such as IGBTs, so that an MOS control semiconductor power converter for outputting a high AC voltage and a high DC voltage can be realized. Since the MOS control semiconductor elements which are series-connected to each other and constitute each of these arms are turned ON, or OFF at the same time in response to a pulse signal controlled by either the PWM control or the PAM control, the DC voltage may be converted into the AC voltage and/or the AC voltage may be converted into the DC voltage.
On the other hand, another technique is opened by which the MOS control semiconductors series-connected to each other, which constitute the respective arms, may be protected from overvoltages. The published abstract of the Japanese Electric Society Industrial Application Department Meeting in 1999, vol. 2, entitled xe2x80x9cSwitching Test of Flat-pack IGBTs connected in Seriesxe2x80x9d, pp. 119-120 describes the following protection technique. That is, the avalanche element is connected between the gate and the collector of the IGBT, while the avalanche element is brought into the conductive state when this avalanche element exceeds a predetermined voltage and thus avalanches. The voltage of the avalanche element is also increased in connection with the increase of the collector voltage the IGBT. When this voltage of the avalanche element exceeds the avalanche voltage of the avalanche element, the current is supplied from the collector of the IGBT to the gate thereof via this avalanche element, so that the gate voltage of the IGBT is increased so as to lower the impedance of the IGBT. As a result, the collector voltage of the IGBT is suppressed in order that the IGBT can be protected from the element destruction (breakdown) by applying the overvoltage to this IGBT. Also, this publication entitled xe2x80x9cSwitching Test for Series-Connection of Planar IGBTsxe2x80x9d, vol. 2 (1999) of the lecture on Japanese Electric Society Industrial Application Department discloses that the MOS control semiconductors can be protected in such a manner that the gate voltage is increased so as to increase the saturated current value.
However, in the above-described publication entitled xe2x80x9cSwitching Test for Series-Connection of Planar IGBTsxe2x80x9d in the published abstract of Japanese Electric Society Industrial Application Department Meeting in 1999, vol. 2, in such a case that the overcurrent is supplied to such an arm under ON state among the arms which constitute the MOS control semiconductor converter, the MOS control semiconductor having the lowest saturated current selected from the MOS control semiconductors which constitute this MOS control semiconductor series-connection limits this overcurrent to the saturated current value of this IGBT. As a consequence, since the MOS control semiconductor having the lowest saturated current limits the current, the impedance thereof is increased and the voltage sharing of this MOS control semiconductor is increased. Thus, the semiconductor element may be destroyed due to the application of the overvoltage.
On the other hand, in the above-described publication entitled xe2x80x9cSwitching Test for Series-Connection of Planar IGBTsxe2x80x9d in the published abstract of Japanese Electric Society Industrial Application Department Meeting in 1999, vol. 2, since such an expensive semiconductor element having a high-voltage withstanding avalanche voltage equivalent to that of such an IGBT to be protected is required.
The present invention has an object to provide a semiconductor power converting apparatus containing such a circuit capable of preventing an application of an overvoltage. That is, in order to protect MOS control semiconductor devices from the overvoltage, when an overcurrent flows through these MOS control semiconductor devices, this circuit can avoid such an operation that the overvoltage is applied to such an MOS control semiconductor having a minimum saturated current among the series-connected MOS control semiconductor devices, while such a semiconductor element having an avalanche voltage equal to the high withstand voltage is not employed.
According to one aspect of the present invention, a semiconductor power converting apparatus, according to an aspect of the present invention, is featured by that while a current is supplied to a gate of an IGBT from a gate driver of an MOS control semiconductor, a gate voltage of such an MOS control semiconductor which is reached to a saturated current is increased higher than a gate voltage obtained under steady ON state, and thus, the saturated current value of this MOS control semiconductor element is increased.
In general, such a relationship as shown in FIG. 2 is established between a collector-to-emitter voltage (will be referred to as a xe2x80x9ccollector voltagexe2x80x9d hereinafter) of an MOS control semiconductor such as an IGBT, and a collector current of this MOS control semiconductor. When the collector voltage is increased at an arbitrary gate-to-emitter voltage (will be referred to as a xe2x80x9cgate voltagexe2x80x9d hereinafter), the collector current is also increased in connection with this operation. When this increased collector current is reached to a certain current value, this collector current does not exceed this reached current value. This maximum current value is referred to as a xe2x80x9csaturated current value.xe2x80x9d The higher the gate voltage is increased, the larger the saturated current value is increased.
As indicated in FIG. 3, while MOS control semiconductors 11 to 14 such as IGBTs having different saturated current values from each other are series-connected to each other and then the series-connected MOS control semiconductors are connected to a voltage source 21, it is so assumed that saturated current values of the respective IGBTs (namely, saturated current value at gate voltages under steady ON states) are defined by IGBT 11 less than IGBT 12 less than IGBT 13 less than IGBT 14. In the case that all of the IGBT series-connected to each other are brought into ON states, a current may flow through this IGBT series-connection at a current increased rate which is determined based upon both a leakage impedance 23 of a wiring line and the voltage source 21. Generally speaking, since a gate voltage of an IGBT is controlled in such a manner that this gate voltage may become a certain gate voltage higher than a threshold value, the IGBT is transferred from an OFF state into an ON state. In this connection, xe2x80x9ca certain gate voltage higher than a threshold valuexe2x80x9d will be referred to as a xe2x80x9csteady ON gate voltagexe2x80x9d hereinafter in this specification.
In the case that a current flowing through the IGBT series-connection indicated in FIG. 3 reaches the saturated current value during the steady ON gate voltage of the IGBT 11 having the lowest saturated current, this IGBT 11 having the lowest saturated current limits this current. As a consequence, since the IGBT 11 limits the current, the impedance thereof is increased. Since a voltage applied to a certain element is equal to a product between an impedance of this element and a current flowing through this element, the collector voltage of the IGBT 11 is increased while the impedance is increased.
However, when the collector voltage of the IGBT under ON state exceeds a previously set value, if the gate circuit owns such a function that the higher the collector becomes, the higher the gate voltage of the IGBT is increased, then the gate voltage of the IGBT 11 becomes higher than the steady ON gate voltage in connection with the increase of the collector voltage of the IGBT 11, so that the saturated current value of the IGBT 11 can be increased up to the saturated current value of the IGBT 12 at the steady ON gate voltage. It should be noted that the previously set value is set within a range defined from the steady OFF voltage and the withstanding voltage of the semiconductor element. When the saturated current value of the IGBT 11 is reached to the saturated current value of the IGBT 12, both the IGBT 11 and the IGBT 12 may limit the current, so that the voltage sharing by the IGBT 11 can be reduced by xc2xd. As a result, in the case that the voltage of the DC voltage source 21 is smaller than the summed value of the IGBT 11 and the IGBT 12, it is possible to avoid such an operation that the semiconductor elements are destroyed, or brought into the breakdown state due to the overvoltage applied to the IGBT.
On the other hand, in such a case that the voltage of the DC voltage source 21 is higher than a total value of the element withstanding voltages of both the IGBT 11 and the IGBT 12, the collector voltages of both the IGBT 11 and the IGBT 12 are further increased. In connection to this collector voltage increase, the gate voltages of both the IGBT 11 and the IGBT 12 are further increased, so that the saturated current values of the IGBT 11 and the IGBT 12 are reached to the saturated current value of the IGBT 13 at the steady ON gate voltage. When the saturated current values of both the IGBT 11 and the IGBT 12 are reached to the saturated current of the IGBT 13, the voltage of the DC power source 21 can be shared by three sets of IGBTs, namely the IGBT 11, the IGBT 12, and the IGBT 13. As a result, if the voltage of the DC power source 21 is lower than the element withstanding voltages of the IGBT 11, the IGBT 12, and the IGBT 13, then it is possible to avoid the element breakdown caused by the application of the overvoltage.
Also, in such a case that the voltage of the DC voltage source 21 is higher than a total value of the element withstanding voltages of the IGBT 11, the IGBT 12, and the IGBT 13, the collector voltages of the IGBT 11, the IGBT 11, and the IGBT 13 are further increased. In connection to this collector voltage increase, the gate voltages of the IGBT 11, the IGBT 12, and the IGBT 13 are further increased, so that the saturated current values of the IGBT 11, the IGBT 12, and the IGBT 13 are reached to the saturated current value of the IGBT 14. When the saturated current values of the IGBT 11, IGBT 12, the IGBT 13 and the IGBT 14 are equal to each other, the voltage of the DC voltage source 21 can be shared by four sets of the IGBTs, namely, the IGBT 11, the IGBT 12, the IGBT 13, and the IGBT 14.
On the other hand, since the series-connection of the IGBT 11 to the IGBT 14 may block the voltage of the voltage source 21 under OFF state, a total value of the element withstanding voltages of the series-connection constructed of the IGBT 11 to the IGBT 14 necessarily exceeds the voltage of the voltage source 21. As a consequence, if the saturated currents of the IGBT 11, the IGBT 12, the IGBT 13, and the IGBT 14 are equal to each other, then the voltage of the DC voltage source 21 can be shared by four sets of the IGBTs, namely, the IGBT 11, the IGBT 12, the IGBT 13, and the IGBT 14. As a result, it is possible to prevent the semiconductor elements from being destroyed due to the application of the overvoltage.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.